In order to fight these monsters, you need to have completed Children of Mah and at least level 80 Slayer. In order to be assigned them for a task, you must have bought the ability from a Slayer Master for 50 Slayer points.

When fighting Nightmares, they primarily use Melee; so protect/deflect from Melee or use of Soul split is suggested. At some points you will notice their hands come together and they hurl a black ball at you. At this point it is advised to switch to protect/deflect from Ranged regardless of what prayer/curse you were using before. Failing to do such can cause you to be hit with 4,000 damage.